Glucane structural unit

Glucan structural unit is the other factor which has major influence on the bioactivities of polysaccharides. Mostly, glucan is the basic structural unit of polysaccharides extracted from plants [113]. The literature strongly indicates that the antitumor and antiviral properties of polysaccharides isolated from medicinal plants/mushrooms associate with their glucan structures [114]. For example, len-tinan from Lentinula edodes, schizophyllan from Schizophyllum commune, which have been used for extensive anticancer treatment and also in many clinical trials, contain glucan units in their mam-chain structures [115,116],... 

Fig. 11.4. Structural unit of the glucane molecule in Botrytis cinerea (or cinereane) showing the concatenation of glncose molecules (Dubourdieu, 1982)...

The relationship between the chemical structure and anti-tumour activity of D-glucans prepared from Grifora umbellata has been investigated. The antitumour D-glucans appear to contain the basic structural unit (29) although the length of the branches, branching frequency, molecular size, and molecular... 

The monosaccharide structural unit conformation and the positions and types of linkages in the chain determine the chain conformation of a polysaccharide. In addition to irregular conformations, regular conformations are known which reflect the presence of at least a partial periodic sequence in the chain. Some typical conformations will be explained in the following discussion, with examples of glucans and some other polysaccharides. 

At ambient relative humidity, starch granules take about 0.2 g of water per 1 g of dry starch from the atmosphere and contain about 17% water (wheat starch is about 13% water and potato starch 18-22% water), without changing the volume of the grains. The process is called imbibition (see Section 7.8.3.1). One molecule of glucose binds 1.5 water molecules. The structural units of glucans contain a total of five oxygen atoms that can interact with water (4-139). 

Single crystals of the D-glucan from Acetobacter xijlinum were prepared in water-methanol solutions at room temperature. Using electron and X-ray diffraction, a hexagonal unit-cell, with a = h = 0.518 nm and c = 2.0 nm, was derived. It was concluded that the chain axes lie parallel to the surface of the lamellar crystals, that there are 2.5 water molecules per /3-D-Glcp residue, and that the structure is similar to that of cellulose hydrate II (Ref. 42). 

Since the 5-position of successive saccharide units occur on the outside surface of the triple-strand molecule it was evident after the discovery of such a model for B-(1-3)-D-xylan that a similar structure would be feasible for e-(l-3)-D-glucan. 

The primary structure of the extracellular polysaccharide from X. aampestris was recently reinvestigated by Jansson et al. (2), and by Melton et al. (8). The chemical repeat unit is a pentasaccharide. The backbone is a 8-1,4 glucan like cellulose and chitin (9,10) with a trisaccharide side-chain on each cellobiose unit (I). 

---